There are two ways to see this assertion is false. First, consider that Direct3D 12 is part of the Windows SDK, which comes with a license agreement (it’s generally located in the
License subdirectory of the installation) that you must agree to before you can use the SDK. The Windows SDK is the minimal piece of Microsoft technology you need to use Direct3D 12, and the Windows SDK license says nothing about any other rendering APIs, nothing about exclusivity, and nothing about restrictions on using Vulkan. It’s pretty clear that you can develop software using that SDK without encumbering yourself in any kind of restriction about rendering API choice.
Second, Windows is not a walled-garden platform. Agreement to the Windows SDK’s license is the first and last legal interaction you can have with Microsoft when developing and shipping software for Windows platforms. There is no other point where they can assert legal leverage to mandate that you not use Vulkan.
So this claim is false.
It seems likely your “source” is either flat-out incorrect, or there is a misunderstanding between yourself and that source. It is theoretically possible that Microsoft, acting as a publisher for a game (and thus legally and financially engaged in a contract with a developer to produce that game) might mandate that a particular game use particular technology, such as D3D12, over other options… Permanently or for a period of exclusivity defined by the contract. Such deals are specific, though: they don’t apply to everybody and as the publisher Microsoft would generally be paying for the right to make such a demand by virtue of the fact that they’re funding the development of the game.
As hard as I try I can’t find anything online giving this the slightest bit of credence and it just seems terribly unlikely, Microsoft does silly things all the time but they’re not completly bereft of sense and I know Cryengine supports both DirectX 12 and Vulkan so at least one vendor can use both without being struck down.
This just sounds like the claim about Visual Studio Express having limitations all over again.